A bigger load than it looks
Water heating is often the second-largest energy use in a US home, after space heating (EIA). DOE puts it at about 18% of home energy use. Showers, laundry and dishes add up.
Off the grid, a standard electric water heater can use about as much energy as the rest of a small, efficient home. It is one of the big loads from lesson 22.
The good news: hot water is easy to store. A full tank is a kind of battery, and that opens up options.
The math of hot water
Water is heavy, and it takes a lot of heat to warm. One gallon weighs about 8.34 lb (3.8 kg). Raising it by 1 °F takes 8.34 BTU.
Temperature rise is how far you heat the water: from the cold supply to the tank setting. Cold water from a well might be 45 °F in the north and 70 °F in the south, so climate changes the answer here too.
Set your daily hot water use and temperature rise. Compare resistance and heat pump energy.
Heat needed: 4.3 kWh per day.
Electric resistance: about 4.5 kWh. Heat pump water heater: about 1.2 to 1.7 kWh.
Assumptions: Water weighs 8.34 lb per gallon; 1 BTU heats 1 lb by 1 °F; 3,412 BTU per kWh. Standby losses not included.
Hot water use varies a lot from home to home. Two people might use roughly 20 to 40 gallons a day, more with long showers and older fixtures. Low-flow fixtures cut this without much change in comfort.
Tanks, tankless and standby loss
A storage water heater keeps a tank of water hot all the time. Heat leaks out of the tank and pipes even when nobody uses water. That is standby loss.
A tankless water heater heats water only as it flows. It has no tank to lose heat. But it must heat water very fast, so it needs a large gas burner or a large electric load at the moment you open the tap. A large electric tankless heater can be a poor fit for an off-grid inverter.
Insulating hot water pipes, and keeping the tank close to where you use hot water, cut losses with either type.
Heat-pump water heaters
A heat-pump water heater uses the same idea as the heat pump in lesson 31. It pulls heat from the air around it and moves it into the tank. DOE says it can be two to three times as efficient as a standard electric water heater.
It has needs of its own:
- It needs space with enough air around it. DOE and ENERGY STAR call for about 1,000 cubic feet (28 m³) of air space.
- It needs a room that stays between about 40 and 90 °F (4 and 32 °C) all year.
- It cools and dries the room it sits in. That is welcome in a hot basement in summer, and costly inside a heated home in winter.
- It is usually slower to recover than a resistance heater after heavy use.
Text description
Left: a section through a utility room with a heat-pump water heater. The heat pump sits on top of the storage tank. Warm room air is drawn in on one side, and cooler, drier air comes out the other. Electricity runs the fan and compressor. Heat taken from the air is moved down into the water in the tank. Cold water comes in and hot water goes out to the house. Right: a bar comparison of the electricity used to heat the same water. A standard electric resistance tank is the full bar. A heat-pump water heater uses about one-half to one-third as much, because it is two to three times as efficient. Notes list its needs: about 1,000 cubic feet (28 cubic meters) of air space, a room that stays between about 40 and 90 °F (4 and 32 °C) all year, it cools and dries its room, and it is slower to recover after heavy use.
Solar water heating
A solar thermal collector is a panel that heats water or another fluid directly with sunlight. It is different from a solar electric panel. The hot fluid heats a storage tank.
- Passive systems move water by natural circulation, called thermosiphon: hot water rises, cool water sinks. They have no pump, but freeze easily.
- Active systems use pumps and controls. In cold climates, most use an antifreeze loop that never touches your drinking water (DOE).
Text description
A simplified section of a house with a solar thermal collector on the south roof. Sunlight heats an antifreeze fluid in the collector. The hot fluid flows down to a coil near the bottom of a storage tank, gives its heat to the water, and the cooler fluid is pumped back up to the collector. A pump and controls run the loop. The antifreeze loop never touches the drinking water. Cold water enters the bottom of the tank, and hot water leaves the top for the taps. A backup heater in the top of the tank covers cloudy weeks and winter. Notes say the system needs a way to dump heat in summer if nobody uses the water, and that freeze, overheating and pressure protection are design work for a professional.
Solar water heating can cover a large share of hot water in sunny climates and much less in cloudy winters. You still need a backup heater. Systems can overheat in summer if nobody uses the water, so they need ways to dump heat.
Some homesteads now pair extra solar electric panels with a heat-pump water heater instead. Fewer moving parts and no freeze worries. Compare both with local numbers before you choose.
Gas, propane and wood-fired water heating
Gas and propane water heaters work without much electricity and recover fast. They burn fuel, so they make carbon monoxide and must be vented correctly. A blocked flue or a backdraft can push exhaust into the home.
Wood-fired water heating uses a coil or water jacket in a stove, or a separate wood-fired boiler. It can make use of heat from a winter fire you already run.
It also carries a serious risk. Water heated in a closed space turns to steam and builds pressure fast. A system without the right pressure relief and design can burst violently. Wood-fired water systems are design work for a qualified professional, not a weekend project.
Scalds, pressure and CO
Every water heater has safety features that must stay in place and working.
- A temperature and pressure relief valve (T&P valve) opens if a tank gets too hot or the pressure too high. Never cap, plug or remove it.
- Hot water scalds. DOE and CPSC suggest a tank setting of 120 °F (49 °C) for most homes, which also saves energy. Higher settings scald faster. If someone in the home has a weak immune system or a chronic lung disease, ask a doctor about the right setting.
- Gas, propane and wood water heaters need working CO alarms nearby (NFPA).
Compare the options
Check
Why should a water heater’s temperature and pressure relief valve never be capped or removed?
Summary
- Hot water is often one of the largest energy loads in a home.
- Energy depends on gallons per day and temperature rise, and climate changes the rise.
- Heat-pump water heaters cut electric use a lot. Solar thermal works best where winters are sunny.
- Burning fuel means venting and CO alarms. Every tank needs a working T&P valve.
Next, in the Food track, you will see how much land it takes to feed a person.
Go deeper: drain-water heat recovery and point-of-use heaters
A drain-water heat recovery unit is a section of copper drain pipe wrapped with a coil. Warm shower water going down the drain preheats the cold water coming in. It needs a vertical drain run and works best when showers are the main hot water use. Small point-of-use heaters at a far sink can cut long pipe runs and the water wasted waiting for hot water. Both need plumbing work and may need a permit.